For the first time, transition metal-based chalcogenides conforming to the definition of high entropy materials, are synthesized, with the multicomponent occupation being utilized on both cationic and anionic sublattices. The pentlandite-structured (Co,Fe,Ni)9S8 and (Co,Fe,Ni)9(S,Se)8 compositions are obtained using a two-stage, solid-state reaction method. Room temperature structural analysis (XRD, SEM, Raman) in both cases indicates the presence of a homogeneous, single-phase, Fm3[combining macron]m structure, with a profound effect of Se addition on the lattice parameters. The obtained materials possess an excellent electrical conductivity of 105 S m-1, and slightly negative Seebeck coefficient values, resulting from their metallic character, combined with a low thermal conductivity of 2.5 W m-1 K-1, especially when compared with conventional analogues. The optical measurements reveal very promising behavior in the UV/vis range. The electrochemical sensitivity towards hydrazine and acetaminophen is also presented, making them potentially interesting for sensor devices. Based on the DFT analysis of various sub-systems, the origins of the observed transport and optical behavior are explained. Furthermore, it is shown that the application of the high-entropy principle to both sublattices simultaneously allows for extensive tailoring of the band structure, allowing these materials to be optimized with respect to the given application, including thermoelectric and photoelectrochemical devices and catalysis, e.g., the hydrogen evolution reaction.